Normal heart function is characterized by a rhythmic contraction of the heart muscle, wherein each contraction is followed by a refractory period and cardiac diastole during which the heart muscle relaxes to be refilled with circulating blood. A diseased heart, however, may experience filling disorders or ineffective contractions under certain conditions that diminish heart and circulatory function leading to heart failure. In response to a diminished heart function, a relatively common practice has been to place electrodes via the cardiac veins on the epicardial surface of the heart's left ventricle, and to then electrically stimulate the area of placement for the purpose of synchronizing heart contractions. Such a technique, however, is not predictably effective and has resulted in no benefit in significant numbers of patients where substantial benefit would have been predicted. Moreover, the possibility of cell damage in the area of electrode placement from observed loss in contractile function has raised additional concerns.
Though several mechanisms are known to contribute to contractions of the heart muscle, they each do so in different ways. As indicated above, one such mechanism involves a direct electrical stimulation of the heart muscle and control of the sequence of muscle activation. Another mechanism for improving muscle function, however, involves the stimulation of sympathetic nerves. More specifically, it is known that norepinephrine (a derivative of adrenaline released from the nervous system nerve endings at the heart) is a potent stimulant of contraction on the heart muscle. It is also known that sympathetic nerves can be electrically stimulated to secrete norepinephrine in response to relatively low intensity stimulation patterns. Importantly, stimulation of the sympathetic nervous system can be efficacious for energizing the heart muscle by causing the release of norepinephrine at low stimulation intensities that do not have any direct effects to electrically stimulate the heart muscle itself.
In light of the above, it is an object of the present invention to provide a system and method that will improve heart function by indirectly stimulating the sympathetic nervous system. Another object of the present invention is to provide a system and method that avoids direct stimulation of the heart muscle while improving heart function with indirect electrical stimulation. Yet another object of the present invention is to improve heart function using low intensity, electrical stimulation patterns during the heart's refractory period that will not adversely affect the heart muscle, or otherwise diminish its local muscle function. Another object of the present invention is to provide a system and method for improving heart function by electrical stimulation that alters the sequence of muscle activation but is also focused on activation of the cardiac sympathetic nervous system, is simple to implement, is easy to use and is comparatively cost effective.